The invention lies in the field of power generation. The invention relates to a continuous-flow steam generator having a combustion chamber for fossil fuel that is followed on the fuel-gas side, through a horizontal gas flue, by a vertical gas flue. The containment walls of the combustion chamber are formed from vertically disposed evaporator tubes gastightly welded to one another.
In a power plant with a steam generator, the energy content of a fuel is utilized for evaporating a flow medium in the steam generator. In such a case, the flow medium is normally carried in an evaporator circuit. The steam supplied by the steam generator may, in turn, be provided, for example, for driving a steam turbine and/or for a connected external process. If the steam drives a steam turbine, a generator or a working machine is usually operated through the turbine shaft of the steam turbine. Where a generator is concerned, the current generated by the generator may be provided for feeding into an interconnected and/or island network.
The steam generator may, in this context, be configured as a continuous-flow steam generator. A continuous-flow steam generator is disclosed in the paper xe2x80x9cVerdampferkonzepte fxc3xcr Benson-Dampferzeugerxe2x80x9d [xe2x80x9cEvaporator concepts for Benson steam generatorsxe2x80x9d] by J. Franke, W. Kxc3x6hler, and E. Wittchow, published in VGB Kraftwerkstechnik 73 (1993), No. 4, p. 352-360. In a continuous-flow steam generator, the heating of steam generator tubes provided as evaporator tubes leads to an evaporation of the flow medium in the steam generator tubes in a single pass.
Continuous-flow steam generators are conventionally configured with a combustion chamber in a vertical form of construction. As such, the combustion chamber is configured for the heating medium or fuel gas to flow through in an approximately vertical direction. In such a case, the combustion chamber may be followed on the fuel-gas side by a horizontal gas flue, a deflection of the fuel-gas stream into an approximately horizontal direction of flow taking place at the transition from the combustion chamber into the horizontal gas flue. However, in general, because of the thermally induced changes in length of the combustion chamber, such combustion chambers require a framework on which the combustion chamber is suspended. The suspension necessitates a considerable technical outlay in terms of the production and assembly of the continuous-flow steam generator, and the outlay is even greater when the overall height of the continuous-flow steam generator is larger. The increase is true, particularly with regard to continuous-flow steam generators that are configured for a steam power output of more than 80 kg/s under full load.
A continuous-flow steam generator is not subject to any pressure limitation, so that fresh-steam pressures well above the critical pressure of water (Pcri=221 bar), where there is still only a slight density difference between the liquid-like and steam-like media, are possible. A high fresh-steam pressure is conducive to high thermal efficiency and, therefore, to low CO2 emissions of a fossil-fired power station that can be fired, for example, with hard coal or else with lignite in solid form as fuel.
A particular problem is presented by the construction of the containment wall of the gas flue or combustion chamber of the continuous-flow steam generator in terms of the tube-wall or material temperatures that occur there. In the subcritical pressure range down to about 200 bar, the temperature of the containment wall of the combustion chamber is determined essentially by the height of the saturation temperature of the water, when wetting of the inner surface of the evaporator tubes can be ensured. Such wetting is achieved, for example, by using evaporator tubes that have a surface structure on their inside. Consideration is given, in particular, to internally ribbed evaporator tubes, of which the use in a continuous-flow steam generator is present in the prior art, for example, from the paper quoted above. These so-called ribbed tubes, that is to say tubes with a ribbed inner surface, have particularly good heat transmission from the tube inner wall to the flow medium.
Experience has shown that it is not possible to avoid the situation, when the continuous-flow steam generator is in operation, where thermal stresses occur between adjacent tube walls of different temperature when these are welded to one another. Such stresses occur, in particular, with regard to the portion of the combustion chamber connecting the combustion chamber to the horizontal gas flue following the combustion chamber, in other words, between evaporator tubes of the outlet region of the combustion chamber and steam generator tubes of the inlet region of the horizontal gas flue. These thermal stresses can markedly reduce the useful life of the continuous-flow steam generator and, in an extreme case, may even give rise to tube cracks.
It is accordingly an object of the invention to provide a fossil-fired continuous-flow steam generator that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that requires a particularly low outlay in terms of production and assembly and, moreover, during the operation of which, keeps low temperature differences at the connection of the combustion chamber to the horizontal gas flue following the combustion chamber. The features apply particularly to the mutually directly or indirectly adjacent evaporator tubes of the combustion chamber and steam generator tubes of the horizontal gas flue following the combustion chamber.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a continuous-flow steam generator, including a combustion chamber for combusting fossil fuel, the combustion chamber having a fuel-gas side, an outlet region, burners, and containment walls formed from vertically disposed evaporator tubes welded to one another in a gastight manner, a horizontal gas flue having an inlet region and being disposed level with respect to the burners of the combustion chamber, a vertical gas flue connected to the fuel-gas side of the combustion chamber through the horizontal gas flue, a connecting portion forming the outlet region of the combustion chamber and the inlet region of the horizontal gas flue, a plurality of the evaporator tubes respectively acted upon in parallel by a flow medium, and a number of the evaporator tubes formed in a loop in the connecting portion.
The continuous-flow steam generator has a combustion chamber with a number of burners disposed level with the horizontal gas flue. A plurality of the evaporator tubes is respectively acted upon in parallel by flow medium. A number of the evaporator tubes are acted upon in parallel by flow medium being led in the form of a loop in a connecting portion that is made of the outlet region of the combustion chamber and the inlet region of the horizontal gas flue.
The invention proceeds from the notion that a continuous-flow steam generator capable of being set up at a particularly low outlay in terms of production and assembly should have a suspension structure capable of being executed in a simple way. At the same time, a framework capable of being set up at a comparatively low technical outlay for the suspension of the combustion chamber can be accompanied by a particularly low overall height of the continuous-flow steam generator. A particularly low overall height of the continuous-flow steam generator can be achieved by the combustion chamber being configured in a horizontal form of construction. For such a purpose, the burners are disposed level with the horizontal gas flue in the combustion chamber wall. Thus, when the continuous-flow steam generator is in operation, the fuel gas flows through the combustion chamber in an approximately horizontal main direction of flow.
Moreover, when the continuous-flow steam generator with the horizontal combustion chamber is in operation, temperature differences should be particularly low at the connection of the combustion chamber to the horizontal gas flue, in order reliably to avoid premature material fatigues as a result of thermal stresses. These temperature differences should be especially low, in particular, between mutually directly or indirectly adjacent evaporator tubes of the combustion chamber and steam generator tubes of the horizontal gas flue, so that material fatigues as a result of thermal stresses are prevented particularly reliably in the outlet region of the combustion chamber and in the inlet region of the horizontal gas flue.
However, when the continuous-flow steam generator is in operation, the inlet portion of the evaporator tubes that is acted upon by flow medium has a comparatively lower temperature than the inlet portion of the steam generator tubes of the horizontal gas flue following the combustion chamber. To be precise, comparatively cold flow medium enters the evaporator tubes in contrast to the hot flow medium that enters the steam generator tubes of the horizontal gas flue. Hence, when the continuous-flow steam generator is in operation, the evaporator tubes are colder in the inlet portion than the steam generator tubes in the inlet portion of the horizontal gas flue. As such, material fatigues resulting from thermal stresses are to be expected at the connection between the combustion chamber and the horizontal gas flue.
However, if preheated flow medium then enters the inlet portion of the evaporator tubes of the combustion chamber, instead of cold flow medium, the temperature difference between the inlet portion of the evaporator tubes and the inlet portion of the steam generator tubes will no longer be as great as would be the case if cold flow medium were to enter the evaporator tubes. If, therefore, the flow medium is led first in a first evaporator tube, which is disposed further away from the connection of the combustion chamber to the horizontal gas flue than a second evaporator tube, and is then introduced into the second evaporator tube, flow medium preheated by firing enters the second evaporator tube when the continuous-flow steam generator is in operation. The complicated connection between a first and a second evaporator tube may be dispensed with if one evaporator tube has an inlet for flow medium in the middle of the containment wall of the combustion chamber. Then, such an evaporator tube can be led first from the top downward and then from the bottom upward in the combustion chamber. Consequently, when the continuous-flow steam generator is in operation, firing causes a preheating of the flow medium to take place in that portion of the evaporator tube that is led from the top downward, before the flow medium enters the so-called inlet portion of the evaporator tubes in the lower region of the combustion chamber. It proves to be particularly beneficial, at the same time, if a number of the evaporator tubes capable of being acted upon in parallel by flow medium are led in the form of a loop in the respective containment wall of the combustion chamber.
In accordance with another feature of the invention, the side walls of the horizontal gas flue and/or of the vertical gas flue are advantageously formed from vertically disposed steam generator tubes welded to one another in a gastight manner and capable of being acted upon in each case in parallel by flow medium.
In accordance with a further feature of the invention, advantageously, in each case, a number of parallel-connected evaporator tubes of the combustion chamber are preceded, with respect to a direction of flow of the flow medium, by a common inlet header system and are followed by a common outlet header system for flow medium. To be precise, such a continuous-flow steam generator allows for reliable pressure compensation between a number of evaporator tubes capable of being acted upon in parallel by flow medium. Accordingly, in each case, all parallel-connected evaporator tubes between the inlet header system and the outlet header system have the same overall pressure loss. As a result, in the case of an evaporator tube heated to a greater extent, the throughput must rise, as compared with an evaporator tube heated to a lesser extent. The characteristic also applies to the steam generator tubes of the horizontal gas flue or of the vertical gas flue that are capable of being acted upon in parallel by flow medium and that are advantageously preceded by a common inlet header system for flow medium and followed by a common outlet header system for flow medium.
In accordance with an added feature of the invention, the evaporator tubes of the end wall of the combustion chamber are advantageously capable of being acted upon in parallel by flow medium and precede the evaporator tubes of the containment walls, which form the side walls of the combustion chamber, on the flow-medium side. The configuration ensures particularly favorable cooling of the highly heated end wall of the combustion chamber.
In accordance with an additional feature of the invention, evaporator tubes of the end wall of the combustion chamber precede other containment walls of the combustion chamber in a direction of the flow-medium.
In accordance with yet another feature of the invention, the tube inside diameter of a number of the evaporator tubes of the combustion chamber is selected as a function of the respective position of the evaporator tubes in the combustion chamber. The evaporator tubes in the combustion chamber can be adapted thereby to a heating profile predeterminable on the fuel-gas side. Due to the influence brought about thereby on the flow through the evaporator tubes, temperature differences of the flow medium at the outlet from the evaporator tubes of the combustion chamber are kept particularly low in a particularly reliable way.
For particularly good heat transmission from the heat of the combustion chamber to the flow medium carried in the evaporator tubes, in accordance with yet a further feature of the invention, a number of evaporator tubes advantageously have in each case, on their inside, ribs that form a multiflight thread. Advantageously, a pitch angle xcex1 between a plane perpendicular to the tube axis and the flanks of the ribs disposed on the tube inside is smaller than 60xc2x0, and, preferably, smaller than 55xc2x0.
To be precise, in a heated evaporator tube configured as an evaporator tube without internal ribbing, a so-called smooth tube, the wetting of the tube wall, necessary for particularly good heat transmission, can no longer be maintained from a specific steam content onward. With a lack of wetting, there may be a tube wall that is dry in places. The transition to such a dry tube wall leads to a so-called heat transmission crisis with an impaired heat transmission behavior, so that, in general, the tube wall temperatures rise particularly sharply at the point. In an internally ribbed evaporator tube, however, as compared with a smooth tube, the heat transmission crisis arises only in the case of a steam mass content greater than 0.9, that is to say just before the end of evaporation. The effect is attributable to the swirl that the flow experiences due to the spiral ribs. Due to the different centrifugal force, the water fraction is separated from the steam fraction and is transported to the tube wall. The wetting of the tube wall is thereby maintained up to high steam contents, so that there are already high flow velocities at the location of the heat transmission crisis. The configuration gives rise, despite the heat transmission crisis, to relatively good heat transmission and, consequently, to low tube wall temperatures.
In accordance with yet an added feature of the invention, a number of evaporator tubes of the combustion chamber advantageously have a device or means for reducing the throughflow of the flow medium. In such a case, it proves particularly beneficial if the device is configured as throttle devices. Throttle devices may, for example, be fittings in the evaporator tubes, which reduce the tube inside diameter at a point within the respective evaporator tube. At the same time, a device or means for reducing the throughflow in a line system that includes a plurality of parallel lines and through which flow medium can be fed to the evaporator tubes of the combustion chamber also prove to be advantageous. In such a case, the line system may also precede an inlet header system of evaporator tubes capable of being acted upon in parallel by flow medium. For example, throttle assemblies may be provided in one line or in a plurality of lines in the line system. Such devices for reducing the throughflow of the flow medium through the evaporator tubes make it possible to adapt the throughput of the flow medium through individual evaporator tubes to the respective heating of these in the combustion chamber. As a result, in addition, temperature differences of the flow medium at the outlet of the evaporator tubes are kept particularly low in a particularly reliable way.
In accordance with yet an additional feature of the invention, the evaporator tubes and the steam generator tubes have fins, adjacent ones of at least one of the evaporator tubes and the steam generator tubes are welded to one another in a gastight manner by the fins, and each of the fins have a fin width selected as a function of a respective position of a corresponding one of the evaporator tubes and the steam generator tubes in at least one of the combustion chamber, the horizontal gas flue, and the vertical gas flue. Adjacent evaporator or steam generator tubes are welded to one another in a gastight manner on their longitudinal sides advantageously through metal bands, also referred to as fins. These fins can be connected fixedly to the tubes even during the tube production process and can form a unit therewith. The unit formed from a tube and fins is also designated as finned tube. The fin width influences the introduction of heat into the evaporator or steam generator tubes. The fin width is, therefore, adapted to a heating profile predeterminable on the flow-gas side, preferably as a function of the position of the respective evaporator or steam generator tubes in the continuous-flow steam generator. The heating profile so predetermined may be a typical heating profile determined from experimental values or else a rough estimation, such as, for example, a stepped heating profile. By the suitably selected fin widths, even when different evaporator or steam generator tubes are heated to a widely differing extent, an introduction of heat into all the evaporator or steam generator tubes can be achieved such that temperature differences of the flow medium at the outlet from the evaporator or steam generator tubes are kept particularly low. As such, premature material fatigues as a result of thermal stresses are reliably prevented. As a result, the continuous-flow steam generator has a particularly long useful life.
In accordance with again another feature of the invention, the horizontal gas flue advantageously has disposed in it a number of superheater heating surfaces that are disposed approximately perpendicularly to the main direction of flow of the fuel gas and the tubes of which are connected in parallel for the throughflow of the flow medium. These superheater heating surfaces, disposed in a suspended form of construction and also designated as bulkhead heating surfaces, are heated predominantly by convection and follow the evaporator tubes of the combustion chamber on the flow-medium side. A particularly favorable utilization of the fuel-gas heat is thereby ensured.
In accordance with again a further feature of the invention, advantageously, the vertical gas flue has a number of convection heating surfaces that are formed from tubes disposed approximately perpendicularly to the main direction of flow of the fuel gas. These tubes of a convection heating surface are connected in parallel for a throughflow of the flow medium. These convection heating surfaces, too, are heated predominantly by convection.
In order, furthermore, to ensure the particularly full utilization of the heat of the fuel gas, the vertical gas flue advantageously has an economizer.
In accordance with again an added feature of the invention, there are provided convection heating surfaces disposed in the vertical gas flue.
In accordance with again an additional feature of the invention, the combustion chamber has a length defined by a distance between the end wall of the combustion chamber and the inlet region of the horizontal gas flue, and the length is at least equal to a burnup length of the fuel in a full-load mode of the continuous-flow steam generator. Advantageously, the burners are disposed on the end wall of the combustion chamber, that is to say on that side wall of the combustion chamber that is located opposite the outflow orifice to the horizontal gas flue. A continuous-flow steam generator so configured can be adapted particularly simply to the burnup length of the fossil fuel. The burnup length of the fossil fuel refers, in this context, to the fuel-gas velocity in the horizontal direction at a specific average fuel-gas temperature, multiplied by the burnup time tA of the flame of the fossil fuel. The maximum burnup length for the respective continuous-flow steam generator is obtained, in such a case, from the steam power output M under the full load of the continuous-flow steam generator, the so-called full-load mode. The burnup time tA of the flame of the fossil fuel is, in turn, the time that, for example, a coaldust grain of average size requires to burn up completely at a specific average fuel-gas temperature.
In accordance with still another feature of the invention, advantageously, the lower region of the combustion chamber is configured as a funnel. As such, when the continuous-flow steam generator is in operation, ash occurring during the combustion of the fossil fuel can be discharged particularly simply, for example, into an ash removal device disposed under the funnel. The fossil fuel may be coal in solid form.
To keep material damage and undesirable contamination of the horizontal gas flue, for example, due to the introduction of high-temperature molten ash, particularly low, the length of the combustion chamber, defined by the distance from the end wall to the inlet region of the horizontal gas flue, is advantageously at least equal to the burnup length of the fossil fuel in the full-load mode of the continuous-flow steam generator. The horizontal length of the combustion chamber will generally amount to at least 80% of the height of the combustion chamber, measured from the funnel top edge, when the lower region of the combustion chamber has a funnel-shaped construction, to the combustion chamber ceiling.
For a particularly beneficial utilization of the combustion heat of the fossil fuel, in accordance with a concomitant feature of the invention, the length L (given in meters) of the combustion chamber is selected as a function of the steam power output M (given in kg/s) of the continuous-flow steam generator under full load, of the burnup time tA (given in seconds) of the flame of the fossil fuel, and of the outlet temperature tBRK (given in xc2x0 C.) of the fuel gas from the combustion chamber. With the given steam power output M of the continuous-flow steam generator under full load, approximately the higher value of the two functions (I) and (II) applies to the length L of the combustion chamber:                               L          ⁡                      (                          M              ,                              t                A                                      )                          =                              (                                          C                1                            +                                                C                  2                                ·                M                                      )                    ·                      t            A                                              (        I        )            
and
L (M, TBRK)=(C3xc2x7TBRK+C4)M+C5(TBRK)2+C6xc2x7TBRK+C7 xe2x80x83xe2x80x83(II), 
where:
C1=8 m/s;
C2=0.0057 m/kg;
C3=xe2x88x921.905xc2x710xe2x88x924(mxc2x7s)/(kgxc2x0 C.);
C4=0.286 (sxc2x7m)/kg;
C5=3xc2x710xe2x88x924 m/(xc2x0 C.)2;
C6=xe2x88x920.842 m/xc2x0 C.; and
C7=603.41 m,
xe2x80x9cApproximatelyxe2x80x9d as defined herein means that a permissible deviation in the length L of the combustion chamber of +20%/xe2x88x9210% from the value defined by the respective function.
The advantages achieved by the invention are, in particular, that, by guiding some evaporator tubes in the form of a loop in the containment wall of the combustion chamber, temperature differences in the immediate vicinity of the connection of the combustion chamber to the horizontal gas flue when the continuous-flow steam generator is in operation are particularly low. Consequently, when the continuous-flow steam generator is in operation, the thermal stresses at the connection of the combustion chamber to the horizontal gas flue that are caused by temperature differences between directly adjacent evaporator tubes of the combustion chamber and steam generator tubes of the horizontal gas flue remain well below the values at which, for example, there is a risk of pipe cracks. It is, therefore, possible to use a horizontal combustion chamber in a continuous-flow steam generator, even at the same time with a comparatively long useful life. Moreover, configuring the combustion chamber for an approximately horizontal main direction of flow of the fuel gas affords a particularly compact form of construction of the continuous-flow steam generator. The configuration makes it possible, when the continuous-flow steam generator is incorporated into a power station with a steam turbine, also to have particularly short connecting pipes from the continuous-flow steam generator to the steam turbine.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a fossil-fired continuous-flow steam generator, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.